﻿<p>An <em>IfcOffsetCurve3D</em> is a curve defined by an offset in 3D space from its <em>BasisCurve</em>.</p>
<blockquote class="extDef">NOTE Definition according to ISO/CD 10303-42:1992<br>
An offset curve 3d is a curve at a constant distance from a basis curve in three-dimensional space. The underlying
curve shall have a well-defined tangent direction at every point. In the case of a composite curve the transition code
between each segment shall be cont same gradient or cont same gradient same curvature. The offset curve at any point
(parameter) on the basis curve is in the direction <em>V x T</em> where <em>V</em> is the fixed reference direction and
<em>T</em> is the unit tangent to the basis curve. For the offset direction to be well defined, <em>T</em> shall not at
any point of the curve be in the same, or opposite, direction as <em>V</em>.
<blockquote class="note">NOTE&nbsp; The offset curve 3d may differ in nature from the basis curve; the offset of a non
self- intersecting curve can be self-intersecting. Care should be taken to ensure that the offset to a continuous curve
does not become discontinuous.</blockquote>
The offset curve 3d takes its parameterization from the basis curve. The offset curve 3d
is parameterized as:
<blockquote><img src="../../../figures/ifcoffsetcurve3d-math1.gif" alt="Math" border="0" width="134" height="22"></blockquote>
<b>T</b> is the unit tangent vector to the basis curve <b>C</b>(<em>u</em>) at
parameter value <em>u</em>, and <em>d</em> is distance. The underlying curve shall be three-dimensional.
</blockquote>
<blockquote class="note">NOTE Entity adapted from <strong>offset_curve_3d</strong> defined in ISO 10303-42</blockquote>
<blockquote class="history">HISTORY&nbsp; New entity in IFC2x</blockquote>
<p class="spec-head">Informal Propositions:</p>
<ol>
<li>At no point on the curve shall ref direction be parallel, or opposite to, the direction of the tangent vector.</li>
</ol>